Rotary pump with exclusively hydrodynamically suspended impeller

ABSTRACT

A pump assembly  1, 33, 200  adapted for continuous flow pumping of blood. In a particular form the pump  1, 200  is a centrifugal pump wherein the impeller  100, 204  is entirely sealed within the pump housing  2, 201  and is exclusively hydrodynamically suspended therein as the impeller rotates within the fluid  105  urged by electromagnetic means external to the pump cavity  106, 203.    
     Hydrodynamic suspension is assisted by the impeller  100, 204  having deformities therein such as blades  8  with surfaces tapered from the leading edges  102, 223  to the trailing edges  103, 224  of bottom and top edges  221, 222  thereof.

FIELD OF THE INVENTION

This invention relates to rotary pumps adapted, but not exclusively, foruse as artificial hearts or ventricular assist devices and, inparticular, discloses in preferred forms a seal-less shaft-less pumpfeaturing open or closed (shrouded) impeller blades with at least partsof the impeller used as hydrodynamic thrust bearings and withelectromagnetic torque provided by the interaction between magnetsembedded in the blades or shroud and a rotating current patterngenerated in coils fixed relative to the pump housing.

BACKGROUND ART

This invention relates to the art of continuous or pulsatile flow rotarypumps and, in particular, to electrically driven pumps suitable for usealthough not exclusively as an artificial heart or ventricular assistdevice. For permanent implantation in a human patient, such pumps shouldideally have the following characteristics: no leakage of fluids into orfrom the bloodstream; parts exposed to minimal or no wear; minimumresidence time of blood in pump to avoid thrombosis (clotting); minimumshear stress on blood to avoid blood cell damage such as haemolysis;maximum efficiency to maximise battery duration and minimise bloodheating; and absolute reliability.

Several of these characteristics are very difficult to meet in aconventional pump configuration including a seal, i.e. with an impellermounted on a shaft which penetrates a wall of the pumping cavity, asexemplified by the blood pumps referred to in U.S. Pat. No. 3,957,389 toRafferty et al., U.S. Pat. No. 4,625,712 to Wampler, and U.S. Pat. No.5,275,580 to Yamazaki. Two main disadvantages of such pumps are firstlythat the seal needed on the shaft may leak, especially after wear, andsecondly that the rotor of the motor providing the shaft torque remainsto be supported, with mechanical bearings such as ball-bearingsprecluded due to wear. Some designs, such as U.S. Pat. No. 4,625,712 toWampler and U.S. Pat. No. 4,908,012 to Moise et al., have overcome theseproblems simultaneously by combining the seal and the bearing into onehydrodynamic bearing, but in order to prevent long residence times theyhave had to introduce means to continuously supply a blood-compatiblebearing purge fluid via a percutaneous tube.

In seal-less designs, blood is permitted to flow through the gap in themotor, which is usually of the brushless DC type, i.e. comprising arotor including permanent magnets and a stator in which an electriccurrent pattern is made to rotate synchronously with the rotor. Suchdesigns can be classified according to the means by which the rotor issuspended: contact bearings, magnetic bearings or hydrodynamic bearings,though some designs use two of these means.

Contact or pivot bearings, as exemplified by U.S. Pat. No. 5,527,159 toBozeman et al. and U.S. Pat. No. 5,399,074 to Nose et al., havepotential problems duo to wear, and cause very high localised heatingand shearing of the blood, which can cause deposition and denaturationof plasma proteins, with the risk of embolisation and bearing seizure.

Magnetic bearings, as exemplified by U.S. Pat. No. 5,350,283 to Nakazekiet al., U.S. Pat. No. 5,326,344 to Bramm et al. and U.S. Pat. No.4,779,614 to Moise et al., offer contactless suspension, but requirerotor position measurement and active control of electric current forstabilisation of the position in at least one direction, according toEarnshaw's theorem. Position measurement and feedback control introducesignificant complexity, increasing the failure risk. Power use by thecontrol current implies reduced overall efficiency. Furthermore, size,mass, component count and cost are all increased.

U.S. Pat. No. 5,507,629 to Jarvik claims to have found a configurationcircumventing Earnshaw's Theorem and thus requiring only passivemagnetic bearings, but this is doubtful and contact axial bearings areincluded in any case. Similarly, passive radial magnetic bearings and apivot point are employed in U.S. Pat. No. 5,443,503 to Yamane.

Prior to the present invention, pumps employing hydrodynamic suspension,such as U.S. Pat. No. 5,211,546 to Isaacson et al. and U.S. Pat. No.5,324,177 to Golding et al., have used journal bearings, in which radialsuspension is provided by the fluid motion between two cylinders inrelative rotation, an inner cylinder lying within and slightly off axisto a slightly larger diameter outer cylinder. Axial suspension isprovided magnetically in U.S. Pat. No. 5,324,177 and by either a contactbearing or a hydrodynamic thrust beating in U.S. Pat. No. 5,211,546.

A purging flow is needed through the journal bearing, a high shearregion, in order to remove dissipated heat and to prevent long fluidresidence time. It would be inefficient to pass all the fluid throughthe bearing gap, of small cross-sectional area, as this would demand anexcessive pressure drop across the bearing. Instead a leakage path isgenerally provided from the high pressure pump outlet, through thebearings and back to the low pressure pump inlet, implying a smallreduction in outflow and pumping efficiency. U.S. Pat. No. 5,324,177provides a combination of additional means to increase the purge flow,namely helical grooves in one of the bearing surfaces, and a smalladditional set of impellers.

U.S. Pat. No. 5,211,546 provides 10 embodiments with various locationsof cylindrical bearing surfaces. One of these embodiments, the third,features a single journal bearing and a contact axial bearing.

Embodiments of the present invention offer a relatively low cost and/orrelatively low complexity means of suspending the rotor of a seal-lessblood pump, thereby overcoming or ameliorating the problems of existingdevices mentioned above.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is disclosed arotary blood pump for use in a heart assist device or like device, saidpump having an impeller suspended in use within a pump housingexclusively by hydrodynamic thrust forces generated by relative movementof said impeller with respect to and within said pump housing.

Preferably at least one of said impeller or said housing includes atleast one deformed surface which, in use, moves relative to a facingsurface on the other of said impeller or said housing thereby to cause arestriction in the form of a reducing distance between the surfaces withrespect to the relative line of movement of said deformed surfacethereby to generate relative hydrodynamic thrust between said impellerand said housing which includes everywhere a localized thrust componentsubstantially and everywhere normal to the plane of movement of saiddeformed surface with respect to said facing surface.

Preferably the combined effect of the localized normal forces generatedon the surfaces of said impeller is to produce resistive forces againstmovement in three translational and two rotational degrees of freedomthus supporting the impeller for rotational movement within said housingexclusively by hydrodynamic forces.

Preferably said thrust forces are generated by blades of said impeller.

More preferably said thrust forces are generated by edges of said bladesof said impeller.

Preferably said edges of said blades are tapered or non-planar so that athrust is created between the edges and the adjacent pump casing duringrelative movement therebetween.

Preferably said edges of said blades are shaped such that the gap at theleading edge of the blade is greater than at the trailing edge and thusthe fluid which is drawn through the gap experiences a wedge shapedrestriction which generates a thrust.

Preferably the pump is of centrifugal type or mixed flow type withimpeller blades open on both front and back faces of the pump housing.

Preferably the front face of the housing is made conical, in order thatthe thrust perpendicular to the conical surface has a radial component,which provides a radial restoring force to a radial displacement or theimpeller axis during use.

Preferably the driving torque of said impeller derives from the magneticinteraction between permanent magnets within the blades of the impellerand oscillating currents in windings encapsulated in the pump housing.

Preferably said blades include magnetic material therein, the magneticmaterial encapsulated within a biocompatible shell or coating.

Preferably said biocompatible shell or coating comprises a diamondcoating or other coating which can be applied at low temperature.

Preferably internal walls of said pump which can come into contact withsaid blades during use are coated with a hard material such as titaniumnitride or diamond coating.

Preferably said impeller comprises an upper conical shroud having saidtaper or other deformed surface therein and wherein blades of saidimpeller are supported below said shroud.

Preferably said impeller further includes a lower shroud mounted inopposed relationship to said upper conical shroud and whereas saidblades are supported within said upper and said lower shroud.

Preferably said deformed surface is located on said impeller.

Preferably said deformed surface is located within said housing.

Preferably forces imposed on said impeller in use, other thanhydrodynamic forces, are controlled by design so that, over apredetermined range of operating parameters, said hydrodynamic thrustforces provide sufficient thrust to maintain said impeller suspended inuse within said pump housing.

Preferably at least one face of the housing is made conical, in orderthat the thrust perpendicular to it has a radial component, whichprovides a radial restoring force to a radial displacement of theimpeller axis. Similarly, an axial displacement toward either the frontor the back face increases the thrust from that face and reduces thethrust from the other face. Thus the sum of the forces on the impellerdue to inertia (within limits), gravity and any bulk radial or axialhydrodynamic force on the impeller can be countered by a restoring forcefrom the thrust bearings after a small displacement of the impellerwithin the housing relative to the housing in either a radial or axialdirection.

In a preferred embodiment, the impeller driving torque derives from themagnetic interaction between permanent magnets within the blades of theimpeller and oscillating currents in windings encapsulated in the pumphousing.

In a further broad form of the invention there is provided a rotaryblood pump having an impeller suspended exclusively hydrodynamically bythrust forces generated by the impeller during movement in use of theimpeller.

Preferably said thrust forces are generated by blades of said impelleror by deformities therein.

More preferably said thrust forces are generated by edges of said bladesof said impeller.

Preferably said edges of said blades are tapered.

In an alternative preferred form said pump is of axial type.

Preferably within a uniform cylindrical section of the pump housing,tapered blade edges form a radial hydrodynamic beating.

In a further broad form of the invention there is provided a rotaryblood pump having a housing within which an impeller acts by rotationabout an axis to cause a pressure differential between an inlet side ofa housing of said pump and an outlet side of the housing of said pump;said impeller suspended exclusively hydrodynamically by thrust forcesgenerated by the impeller during movement in use of the impeller.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, withreference to the accompanying drawings, wherein:

FIG. 1 is a longitudinal cross-sectional view of a preferred embodimentof the invention;

FIG. 2 is a cross-sectional view taken generally along the line Z-Z ofFIG. 1;

FIG. 3A is a cross-sectional view of an impeller blade taken generallyalong the line A-A of FIG. 2;

FIG. 3B is an enlargement of the blade-pump housing interface portion ofFIG. 3A;

FIG. 3C is all alternative impeller blade shape;

FIGS. 4A, B, C illustrate various possible locations of magnet materialwithin a blade;

FIGS. 5A, B and C are left-hand end views of possible winding geometriestaken generally along the line S-S of FIG. 1;

FIG. 6 is a diagrammatic cross-sectional view of an alternativeembodiment of the invention as an axial pump;

FIG. 7 is an exploded, perspective view of a centrifugal pump assemblyaccording to a further embodiment of the invention;

FIG. 8 is a perspective view of the impeller of the assembly of FIG. 7;

FIG. 9 is a perspective, cut away view of the impeller of FIG. 8 withinthe pump assembly of FIG. 7;

FIG. 10 is a side section indicative view of the impeller of FIG. 8;

FIG. 11 is a detailed view in side section of edge portions of theimpeller of FIG. 10;

FIG. 12 is a block diagram of an electronic driver circuit for the pumpassembly of FIG. 7;

FIG. 13 is a graph of head versus flow for the pump assembly of FIG. 7;

FIG. 14 is a graph of pump efficiency versus flow for the pump assemblyof FIG. 7;

FIG. 15 is a graph of electrical power consumption versus flow for thepump assembly of FIG. 7;

FIG. 16 is a plan, section view of the pump assembly showing a volutearrangement according to a preferred embodiment;

FIG. 17 is a plan, section view of a pump assembly showing analternative volute arrangement;

FIG. 18 is a plan view of an impeller according to a further embodimentof the invention;

FIG. 19 is a plan view of an impeller according to a further embodimentof the invention;

FIG. 20 is a perspective view or an impeller according to a furtherembodiment of the invention;

FIG. 21 is a perspective view of an impeller according to yet a furtherembodiment of the invention;

FIG. 22 is a perspective, partially cut away view of an impelleraccording to yet a further embodiment of the intention;

FIG. 23 is a top, perspective view of the impeller of FIG. 22;

FIG. 24 is a perspective view of the impeller of FIG. 22 with its topshroud removed;

FIG. 25 illustrates an alternative embodiment wherein the deformedsurface is located on the pump housing; and

FIG. 26 illustrates a further embodiment wherein deformed surfaces arelocated both on the impeller and on the housing.

FIG. 27 illustrates diagrammatically the basis of operation of the“deformed surfaces” utilised for hydrodynamic suspension of embodimentsof the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The pump assemblies according to various preferred embodiments to bedescribed below all have particular, although not exclusive, applicationfor implantation in a mammalian body so as to at least assist, if nottake over, the function of the mammalian heart. In practice this isperformed by placing the pump assembly entirely within the body of themammal and connecting the pump between the left ventricle and the aortaso as to assist left side heart function. It may also be connected tothe right ventricle and pulmonary artery to assist the right side of theheart.

In this instance the pump assembly includes an impeller which is fullyseared within the pump body and so does not require a shaft extendingthrough the pump body to support it. The impeller is suspended, in use,within the pump body by the operation of hydrodynamic forces imparted asa result of the interaction between the rotating impeller, the internalpump walls and the fluid which the impeller causes to be urged from aninlet of the pump assembly to an outlet thereof.

A preferred embodiment of the invention is the centrifugal pump 1, atdepicted in FIGS. 1 and 2, intended for implantation into a human, inwhich case the fluid referred to below is blood. The pump housing 2, canbe fabricated in two parts, a front part 3 in the form of a housing bodyand a back part 4 in the form of a housing cover, with a smooth jointherebetween, for example at 5 in FIG. 1. The pump 1 has an axial inlet6 and a tangential outlet 7. The rotating part 100Q is of very simpleform, comprising only blades 8 and a blade support 9 to hold thoseblades fixed relative to each other. The blades may be curved asdepicted in FIG. 2, or straight, in which case they can be either radialor back-swept, i.e. at an angle to the radius. This rotating part 100will hereafter be called the impeller 100, but it also serves as abearing component and as the rotor of a motor configuration as to befurther described below whereby a torque is applied by electromagneticmeans to the impeller 100. Note that the impeller has no shaft and thatthe fluid enters the impeller from the region of its axis RR. Some ofthe fluid passes in front of the support cone 9 and some behind it, sothat the pump 1 can be considered of two-sided open type, as compared toconventional open centrifugal pumps, which are only open on the frontside. Approximate dimensions found adequate for the pump 1 to perform asa ventricular assist device, when operating at speeds in the range 1,500rpm to 4,000 rpm, are outer blade diameter 40 mm, outer housing averagediameter 60 mm, and housing axial length 40 mm.

As the blades 8 move within the housing, some of the fluid passesthrough the gaps, much exaggerated in FIGS. 1 and 3, between the bladeedges 101 and the housing front face 10 and housing back face 11. In allopen centrifugal pumps, the gaps are made small because this leakageflow lowers the pump hydrodynamic efficiency. In the pump disclosed inthis embodiment, the gaps are made slightly smaller than is conventionalin order that the leakage flow can be utilised to create a hydrodynamicbearing. For the hydrodynamic forces to be sufficient, the blades mayalso be tapered as depicted in FIGS. 3A and 3B, so that the gap 110 islarger at the leading edge 102 of the blade 8 than at the trailing edge103 thereby providing one example of a “deformed surface” as describedelsewhere in this specification. The fluid 105 which passes through thegap thus experiences a wedge shaped restriction which generates athrust, as described in Reynolds' theory of lubrication (see, forexample, “Modern Fluid Dynamics, Vol. 1 Incompressible Flow”, by N.Curle and H. J. Davies, Van Nostrand, 1968). For blades considerablythinner than their axial length, the thrust is proportional to thesquare of the blade thickness at the edge, and thus in this embodimentthick blades are favoured, since if the proportional of the pump cavityfilled by blades is constant, then the net thrust force will beinversely proportional to the number of blades. However, the blade edgescan be made to extend as tails from thin blades as depicted in FIG. 3Cin order to increase the blade area adjacent the walls.

In one particular form, the tails join adjacent blades so as to form acomplete shroud with wedges or tapers incorporated therein. An exampleof a shroud design as well as other variations on the blade structurewill be described later in this specification.

For manufacturing simplicity, the housing front face 10 can be madeconical, with an angle of around 45° so that it provides both axial andradial hydrodynamic forces. Other angles are suitable that achieve thefunctional requirements of this pump including the requirements for bothaxial and radial hydrodynamic forces.

Other curved surfaces are possible provided both axial and radialhydrodynamic forces can be produced as a result of rotation of theblades relative to the housing surfaces.

The housing back face 11 can include a roughly conical extension 12pointing into the pump cavity 106, to eliminate or minimise the effectof the flow stagnation point oil the axis of the back housing.

Alternatively extension 12 can resemble an impeller eye to make the flowmixed.

In this preferred embodiment, for manufacturing simplicity and foruniformity in the flow axial direction RR the housing back face 11 ismade flat over the bearing surfaces, i.e. under the blade edges. Withthis the case, a slacker tolerance on the alignment between the axes ofthe front, part 3 and back part 4 of the housing 2 is permissible. Analternative is to make the back face 11 conical at the bearing surfaces,with taper in the opposite direction to the front face 10, so that thehydrodynamic forces from the back face will also have radial components.Tighter tolerance on the axes alignment would then be required, and someof the flow would have to undergo a reversal in its axial direction.Again a roughly conical extension (like 12) will be needed. There may besome advantage in making the housing surfaces and blade edgesnon-straight, with varying tangent angle, although this will imposegreater manufacturing complexity.

There are several options for the shape of the taper, but in thepreferred embodiment the amount of material removed simply varieslinearly or approximately linearly across the blade. For the back face,the resulting blade edges are then planes at a slight inclination to theback face. For the front face, the initial blade edges are curved andthe taper only removes a relatively small amount of material so theystill appear curved. Alternative taper shapes can include a step in theblade edge, though the corner in that step would represent a stagnationline posing a thrombosis risk.

For a given minimum yap, at the trailing blade edge, the hydrodynamicforce is maximal if the gap at the leading edge is approximately doublethat at the trailing edge. Thus the taper, which equals the leading edgegap minus the trailing edge gap, should be chosen to match a nominalminimum gap, once the impeller has shifted towards that edge. Dimensionswhich have been found to give adequate thrust forces are a taper ofaround 0.05 mm for a nominal minimum gap of around 0.05 mm, and anaverage circumferential blade edge thickness of around 6 mm for 4blades. For the front face, the taper is measured within the planeperpendicular to the axis. The axial length of the housing between thefront and back faces at any position should then be made about 0.2 mmgreater than the axial length of the blade, when it is coaxial with thehousing, so that the minimum gaps are both about 0.1 mm axially when theimpeller 100 is centrally positioned within the housing 2. Then, forexample, if the impeller shifts axially by 0.05 mm, the minimum gapswill be 0.05 mm at one face and 0.15 mm at the other face. The thrustincreases with decreasing gap and would be much larger from the 0.05 mmgap than from the 0.15 mm gap, about 14 times larger for the abovedimensions. Thus there is a net restoring force away from the smallergap.

Similarly, for radial shifts of the impeller the radial component of thethrust from the smaller gap on the conical housing front face wouldoffer the required restoring radial force. The axial component of thatforce and its torque on the impeller would have to be balanced by anaxial force and torque from the housing back face, and so the impellerwill also have to shift axially and tilt its axis to be no longerparallel with the housing axis. Thus as the person moves and the pump isaccelerated by external forces, the impeller will continually shift itsposition and alignment, varying the gaps in such a way that the totalforce and torque on the impeller 100 match that, demanded by inertia.The gaps are so small, however, that the variation in hydrodynamicefficiency will be small, and the pumping action of the blades will beapproximately the same as when the impeller is centrally located.

While smaller gaps imply greater hydrodynamic efficiency and greaterbearing thrust forces, smaller gaps also demand tighter manufacturingtolerances, increase frictional drag on the impeller, and impose greatershear stress an the fluid. Taking these points in turn, for the above0.05 mm tapers and gaps, tolerances of around 0.005 mm are needed, whichimposes some cost penalty but is achievable. A tighter tolerance isdifficult, especially if the housing is made of a plastic, given thechanges in dimension caused by temperature and possible absorption offluid by plastic materials which may be in contact with the blood suchas Acrylic of polyurethane. The frictional drag for the above gapsproduces much smaller torque than the typical motor torque. Finally, toestimate the shear stress, consider a rotation speed of 3,000 rpm and atypical radius of 15 mm, at which the blade speed is 4.7 ms⁻¹ and theaverage velocity shear for an average gap of 0.075 mm is 6.2×10⁴ s⁻¹.For blood of dynamic viscosity 3.5×10⁻³ kgm⁻¹s⁻¹, the average shearstress would be 220 Nm⁻². Other prototype centrifugal blood pumps withclosed blades have found that slightly larger gaps, e.g. 0.15 μm, areacceptable for haemolysis. A major advantage of the open blades of thepresent invention is that a fluid element that does passing through ablade edge gap will have very short residence time in that gap, around2×10⁻³ S, and the fluid element will most likely be swept though thepump without passing another blade edge.

With particular reference to FIGS. 3A and 3B typical working clearancesand working movement for the impeller 8 with respect to the upper andlower housing surfaces 10, 11 is of the order of 100 microns clearanceat the top and at the bottom. In use gravitational and other forces willbias the impeller 8 closer to one or other of the housing wallsresulting, typically in a clearance at one interface of the order of 50microns and a corresponding larger clearance at the other interface ofthe order of 150 microns. In use, likely maximum practical clearanceswill range from 300 microns down to 1 micron.

Typical restoring forces for a 25 gram rotor mass spinning at 2200 rpmare 1.96 Newtons at a 20 micron clearance extending to 0.1 Newtons at an80 micron clearance.

To minimise the net force required of the hydrodynamic bearings, the netaxial and radial hydrodynamic forces on the impeller from the bulk fluidflow should be minimised, where “bulk” here means other than from thebearing thrust surfaces.

The radial force on the impeller depends critically on the shape of theoutput flow collector or volute 13. The shape should be designed tominimise the radial impeller force over the desired range of pumpspeeds, without excessively lowering the pump efficiency. The optimalshape will have a roughly helical perimeter between the “cutwater” andoutlet. The radial force can also be reduced by the introduction of aninternal division in the volute 13 to create a second output flowcollector passage, with tongue approximately diametrically opposite tothe tongue of the first passage.

An indicative plan view of impeller 100 relative to housing 2 is shownin FIG. 2 having a concentric volute 13.

FIG. 17 illustrates the alternative volute arrangement comprising asplit volute created by volute barrier 107 which causes volute 108 in afirst hemisphere of the housing 2 to split into first half volute 109and second half volute 110 over the second hemisphere. The hemispheresare defined respectively on each side of a diameter of the housing 2which passes through or near exit point 111 of outlet 7.

In alternative forms concentric volutes can be utilised, particularlywhere specific speed is relatively low.

In a further particular form a vaneless diffuser may also reduce theradial force.

In regard to the bulk hydrodynamic axial force, if the bladecross-section is made uniform in the axial direction along therotational axis, apart from the conical front edge, then the pressureacting on the blade surface (excluding the bearing edges) will have noaxial component. This also simplifies the blade manufacture. The bladesupport cone 9 must then be shaped to minimise axial thrust on theimpeller and minimise disturbance to the flow over the range of speeds,while maintaining sufficient strength to prevent relative blademovement. The key design parameter affecting the axial force is theangle of the cone. The cone is drawn in FIG. 1 as having the sameinternal diameter as the blades, which may aid manufacture. However, thecone could be made with larger or smaller internal diameter to theblades. There may be advantage in using a non-axisymmetric support“cone” e.g. with larger radius on the trailing surface of a blade thanthe radius at the leading surface of the next blade. If the blades aremade with non-uniform cross-section to increase hydrodynamic efficiency,then any bulk hydrodynamic axial force on them can be balanced byshaping the support cone to produce an opposite bulk hydrodynamic axialforce on it.

Careful design of the entire pump, employing computational fluiddynamics, is necessary to determine the optimal shapes of the blades 8,the volute 13, the support cone 9 and the housing 2, in order tomaximise hydrodynamic efficiency while keeping the bulk fluidhydrodynamic forces, shear and residence times low. All edges and thejoins between the blades and the support cone should be smoothed.

The means or providing the driving torque on the impeller 100 of thepreferred embodiment of the invention is to encapsulate permanentmagnets 14 in the blades 8 of the impeller 100 and to drive them with arotating magnetic field pattern from oscillating currents in windings 15and 16, fixed relative to the housing 2. Magnets of high remanence suchas sintered rare-earth magnets should be used to maximise motorefficiency. The magnets can be aligned axially but greater motorefficiency is achieved by tilting the magnetisation direction to anangle of around 15° to 30° outwards from the inlet axis, with 22.5° tiltsuitable for a body of conical angle 45°. The magnetisation directionmust alternate in polarity for adjacent blades. Thus there must be aneven number of blades. Since low blade number is preferred for thebearing force, and since two blades would not have sufficient bearingstiffness to rotation about an axis through the blades and perpendicularto the pump housing (unless the blades are very curved), four blades arerecommended. A higher number of blades, for example 6 or 8 will alsowork.

Some possible options for locating the magnets 14 within the blades 8are shown in FIG. 4. The most preferred which is depicted in FIG. 4A, isfor the blade to be made of magnet material apart from a biocompatibleshell or coating to prevent fluid corroding the magnets and to preventmagnet material (which may be toxic) entering the blood stream. Thecoating should also be sufficiently durable especially at blade cornersto withstand rubbing during start-up or during inadvertent bearing touchdown.

In one particular form the inside walls of the pump housing 2 are alsocoated with a biologically compatible and wear resistant material suchas diamond coating or titanium nitride so that wear on both of thetouching surfaces is minimised.

An acceptable coating thickness is approximately 1 micron.

A suitable impeller manufacturing method is to die-press the entireimpeller, blades and support cone, as a single axially aligned magnet.The die-pressing is much simplified if near axially uniform blades areused (blades with an overhang such as in FIG. 3C are precluded). Duringpressing, the crushed rare-earth particles must be aligned in an axialmagnetic field. This method of die-pressing with parallel alignmentdirection is cheaper for rare-earth magnets, although it producesslightly lower remanence magnets. The tolerance in die-pressing is poor,and grinding of the tapered blade edges is required. Then the magnetimpeller can be coated, for example by physical vapour deposition, oftitanium nitride for example, or by chemical vapour deposition, of athin diamond coating or a teflon coating.

In an alternative form the magnet material can be potted in titanium ora polymeric housing which is then, in turn, coated with a biologicallycompatible and tough material such as diamond coating or titaniumnitride.

Finally, to create the alternating blade polarity the impeller must beplaced in a special pulse magnetisation fixture, with an individual coilsurrounding each blade. The support cone of a die-pressed magnetimpeller acquires some magnetisation near the blades, with negligibleinfluence.

Alternative magnet locations are sketched in FIG. 4B and FIG. 4 c inwhich quadrilateral or circular cross-section magnets 14 are insertedinto the blades. Sealing and smoothing of the blade edges over theinsertion holes is then required to reinstate the taper.

All edges in the pump should be radiused and surfaces smoothed to avoidpossible damage to formed elements of the blood.

The windings 15 and 16 of the preferred embodiment are slotless orair-gap windings with the same pole number as the impeller, namely fourpoles in the preferred embodiment. A ferromagnetic iron yoke 17 ofconical form for the front winding and an iron ferromagnetic yoke 18 ofannular form for the back winding may be placed on the outside of thewindings to increase the magnetic flux densities and hence increasemotor efficiency. The winding thicknesses should be designed for maximummotor efficiency, with the sum of their axial thicknesses somewhat lessthan but comparable to the magnet axial length. The yokes can be made ofsolid ferromagnetic material such as iron. To reduce “iron” losses, theyokes 17 can be laminated, for example in layers or by helically windingthin strip, or can be made of iron/powder epoxy composite. The yokesshould be positioned such that there is zero net axial magnetic force onthe impeller when it is positioned centrally in the housing. Themagnetic force is unstable and increases linearly with axialdisplacement of the impeller away from the central position, with thegradient being called the negative stiffness of the magnetic force. Thisunstable magnetic force must be countered by the hydrodynamic bearings,and so the stiffness should be made as small as possible. Choosing theyoke thickness such that the flux density is at the saturation levelreduces the stiffness and gives minimum mass. An alternative can be tohave no iron yokes, completely eliminating the unstable axial magneticforce, but the efficiency of such designs may be lower and the magneticflux density in the immediate vicinity of the pump may violate safetystandards and produce some tissue heating. In any case, the stiffness isacceptably small for slotless windings with the yokes present. Anotheralternative would be to insert the windings in slots in laminated ironstators which would increase motor efficiency and enable use of lessmagnet material and potentially lighter impeller blades. However, theunstable magnetic forces would be significant for such slotted motors.Also, the necessity for fat blades to generate the required bearingforces in this embodiment allows room for large magnets, and so slotlesswindings are chosen in the preferred embodiment.

Instead of determining the yoke positions so that the impeller has zeromagnetic axial force in the central position, it may be possible toprovide a bias axial magnetic force on the impeller, which cancounteract other forces such as any average bulk hydrodynamic axialforce. In particulars by ensuring a net axial force into the conicalbody, the thrust bearings on the cover surface can be made superfluous.However, such a bias would demand greater average thrust forces, smallergaps and increased blood damage, and so the recommended goal is to, zeroboth the magnetic and bulk hydrodynamic axial forces on the impellerwhen centrally positioned.

The overall design requirement for exclusive hydrodynamic suspensionrequires control of the external force balance to make the relativemagnitude of hydrodynamic thrust sufficient to overcome the externalforces. Typical external forces include gravitational forces and netmagnetic forces arising as a result of the motor drive.

There are many options for the winding topology and number of phases.FIG. 5A depicts the preferred topology for the body winding 15, viewedfrom the inlet axis.

The cover winding 16 looks similar but the coils need not avoid theinlet tube and so they appear more triangular in shape. The body windinghas a more complex three-dimensional shape with bends at the ends of thebody cone section. Each winding consists of three coils. Each coil ismade from a number of turns of an insulated conductor such as copperwith the number of turns chosen to suit the desired voltage. The coilside mid-lines span an angle of about 50°-100° at the axis when thecoils are in position. The coils for body and cover are aligned axiallyand the axially adjacent coils are connected in either parallel orseries connection to form one phase of the three phase winding. Parallelconnection offers one means of redundancy in that if one coil fails, thephase can still carry current through the other coil. In parallelconnection each of the coil and body winding has a neutral pointconnection as depicted in FIG. 5A, whereas in series connection, onlyone of the windings has a neutral point.

An alternative three phase winding topology, depicted in FIG. 5B, usesfour coils per phase for each of the body and cover windings, with eachcoil wrapping around the yoke, a topology called a “Gramm ring” winding.

Yet another three phase winding topology, depicted in FIG. 5 c, uses twocoils per phase for each of the body and cover windings, and connectsthe coil sides by azimuthal end-windings as is standard motor windingpractice. The coils are shown tilted to approximately follow the bladecurvature, which can increase motor efficiency, especially for the phaseenergising strategy to be described below in which only one phase isenergised at a time. The winding construction can be simplified bylaying the coils around pins protruding from a temporary former, thepins shown as dots in 2 rings of 6 pins each in FIG. 5C. The coils arelabelled alphabetically in the order in which they would be layed, coilsa and d for phase A, b and e for phase B, and c and f for phase C.Instead of or as well as pins, the coil locations could be defined bythin fins, running between the pins in FIG. 5C, along the boundarybetween the coils. The coil connections depicted in FIG. 5C are thoseappropriate for the winding nearest the motor terminals for the case ofseries connection, with the optional lead from the neutral point on theother winding included.

The winding topologies depicted in FIGS. 5B and C allow the possibilityof higher motor efficiency but only if significantly higher coil mass isallowed, and since option FIG. 5A is more compact and simpler tomanufacture, it is the preferred option. Material ribs between the coilsof option FIG. 5A can be used to stiffen the housing.

Multi-stranded flexible conductors within a suitable biocompatible cablecan be used to connect the rotor windings to a motor controller. Theenergisation of the three phases can be performed by a standardsensorless controller, in which two out of six semiconducting switchesin a three phase bridge are turned on at any one time. Alternatively,because of the relatively small fraction of the impeller cross-sectionoccupied by magnets, it may be slightly more efficient to only activateone of the three phases at a time, and to return the current by aconductor from the neutral point in the motor. Careful attention must bepaid to ensure that the integrity of all conductors and connections isfailsafe.

In the preferred embodiment, the two housing components 3 and 4 are madeby injection moulding from non-electrically conducting plastic materialssuch as Lexan polycarbonate plastic. Alternatively the housingcomponents can be made from ceramics. The windings and yokes are ideallyencapsulated within the housing during fabrication moulding. In thisway, the separation between the winding and the magnets is minimised,increasing the motor efficiency, and the housing is thick, increasingits mechanical stiffness. Alternatively, the windings can be positionedoutside the housing, of thickness at least around 2 mm for sufficientstiffness.

If the housing material plastic is hygroscopic or if the windings areoutside the housing, it may be necessary to first enclose the windingsand yoke in a very thin impermeable shell. Ideally the shell should benon-conducting (such as ceramic or plastic), but titanium of around 0.1mm to 0.2 mm thickness would give sufficiently low eddy losses.Encapsulation within such a shell would be needed to prevent windingmovement.

Alternatively, the housing components 3 and 4 may be made from abiocompatible metallic material of low electrical conductivity, such asTi-6A1-4V. To minimize the eddy current loss, the material must be asthin as possible, e.g. 0.1 mm to 0.5 mm, wherever the materialexperiences high alternating magnetic flux densities, such as betweenthe coils and the housing inner surfaces 10 and 11.

The combining of the motor and bearing components into the impeller inthe preferred embodiment provides several key advantages. The rotorconsequently has very simple form, with the only cost of the bearingbeing tight manufacturing tolerances. The rotor mass is very low,minimising the bearing force needed to overcome weight. Also, with thebearings and the motor in the same region of the rotor, the bearingsforces are smaller than if they had to provide a torque to supportmagnets at an extremity of the rotor.

A disadvantage of the combination of functions in the impeller is thatits design is a coupled problem. The optimisation should ideally linkthe fluid dynamics, magnetics and bearing thrust calculations. Inreality, the blade thickness can be first roughly sized to give adequatemotor efficiency and sufficient bearing forces with a safety-margin.Fortuitously, both requirements are met for four blades of approximateaverage circumferential thickness 6 mm or more. The housing, blade, andsupport cone shapes can then be designed using computational fluiddynamics, maintaining the above minimum average blade thickness. Finallythe motor stator, i.e. winding and yoke, can be optimised for maximummotor efficiency.

FIG. 6 depicts an alternative embodiment of the invention as an axialpump. The pump housing is made of two parts, a front part 19 and a backpart 20, joined for example at 21. The pump has an axial inlet 22 andaxial outlet 23. The impeller comprises only blades 24 mounted on asupport cylinder 25 of reducing radius at each end. An important featureof this embodiment is that the blade edges are tapered to generatehydrodynamic thrust forces which suspend the impeller. These forcescould be used for radial suspension alone from the straight section 26of the housing, with some alternative means used for axial suspension,such as stable axial magnetic forces or a conventional tapered-land typehydrodynamic thrust bearing. FIG. 6 proposes a design which uses thetapered blade edges to also provide an axial hydrodynamic bearing. Thehousing is made with a reducing radius at its ends to form a front face27 and a back face 2 a from which the axial thrusts can suspend themotor axially. Magnets are embedded in the blades with blades havingalternating polarity and four blades being recommended. Iron in theouter radius of the support cylinder 25 can be used to increase themagnet flux density. Alternatively, the magnets could be housed in thesupport cylinder and iron could be used in the blades. A slotlesshelical winding 29 is recommended, with outward bending end-windings 30at one end to enable insertion of the impeller and inward bendingwindings 31 at the other end to enable insertion of the winding into acylindrical magnetic yoke 32. The winding can be encapsulated in theback housing part 20.

Third Embodiment

With reference to FIGS. 7 to 15 inclusive there is shown a furtherpreferred embodiment of the pump assembly 200.

With particular reference initially to FIG. 1 the pump assembly 200comprises a housing body 201 adapted for bolted connection to a housingcover 202 and so as to define a centrifugal pump cavity 203 therewithin.

The cavity 203 houses an impeller 204 adapted to receive magnets 205within cavities 206 defined within blades 207. As for the first;embodiment the blades 207 are supported from a support cone 208.

Exterior to the cavity 203 but forming part of the pump assembly 2030there is located a body winding 209 symmetrically mounted around inlet210 and housed between the housing body 201 and a body yoke 211.

Also forming part of the pump assembly 200 and also mounted external topump cavity 203 is cover winding 212 located within winding cavity 213which, in turn, is located within housing cover 202 and closed by coveryoke 214.

The windings 212 and 209 are supplied from the electronic controller ofFIG. 12 as for the first embodiment the windings are arranged to receivea three phase electrical supply and so as to set up a rotating magneticfield within cavity 203 which exerts a torque on magnets 205 within theimpeller 204 so as to urge the impeller 204 to rotate substantiallyabout central axis TT of cavity 203 and in line with the longitudinalaxis of inlet 210. The impeller 204 is caused to rotate so as to urgefluid (in this case blood) around volute 215 and through outlet 216.

The assembly is bolted together in the manner indicated by screws 217.The yokes 211, 214 are held in place by fasteners 218. Alternatively,press fitting is possible provided sufficient integrity of seal can bemaintained.

FIG. 8 shows the impeller 204 of this embodiment and clearly shows thesupport cone 208 from which the blades 207 extend. The axial cavity 219which is arranged, in use, to be aligned with the longitudinal axis ofinlet 210 and through which blood is received for urging by blades 207is clearly visible.

The cutaway view of FIG. 9 shows the axial cavity 219 and also themagnet cavities 206 located within each blade 207. The preferred conestructure 220 extending from housing cover 202 aligned with the axis ofinlet 210 and axial cavity 219 of impeller 204 is also shown.

FIG. 10 is a side section, indicative view of the impeller 204 definingthe orientations of central axis FE, top taper edge DL) and bottom taperedge BB, which tapers are illustrated in FIG. 11 in side section view.

FIG. 11A is a section of a blade 207 of impeller 204 taken through planeDD as defined in FIG. 10 and shows the top edge 221 to be profiled froma leading edge 223 to a trailing edge 224 as follows; central portion227 comprises an ellipse with centre on the dashed midline having asemi-major axis of radius 113 mm and a semi-minor axis of radius 80 mmand then followed by leading conical surface 225 and trailing conicalsurface 226 on either side thereof as illustrated in FIG. 11A. Theleading surface 225 has radius 0.05 mm less than the trailing surface226. This prescription is for a taper which can be achieved by agrinding wheel, but many alternative prescriptions could be devised togive a taper of similar utility.

The leading edge 223 is radiused as illustrated.

FIG. 11B illustrates in cross-section the bottom edge 222 of blade 207cut along plane BB of FIG. 10.

The bottom edge includes cap 228 utilised for sealing magnet 205 withincavity 206.

In this instance substantially the entire edge comprises a straighttaper with a radius of 0.05 mm at leading edge 229 and a radius of 0.25mm at trailing edge 230.

The blade 207 is 6.0 mm in width excluding the radii at either end.

FIG. 12 comprises a block diagram of the electrical controller suitablefor driving the pump assembly 200 and comprises a three phasecommutation controller 232 adapted to drive the windings 209, 212 of thepump assembly. The commutation controller 232 determines relative phaseand frequency values for driving the windings with reference to setpoint speed input 233 derived from physiological controller 234 which,in turn, receives control inputs 235 comprising motor current input andmotor speed (derived from the commutation controller 232), patient bloodflow 236, and venous oxygen saturation 237. The pump blood flow can beapproximately inferred from the motor speed and current via curve-fittedformulae.

FIG. 13 is a graph of pressure against flow for the pump assembly 200where the fluid pumped is 18% glycerol for impeller rotation velocityover the range 1500 RPM to 2500 RPM. The 18% glycerol liquid is believedto be a good analogue for blood under certain circumstances, for examplein the housing gap.

FIG. 14 graphs pump efficiency against flow for the same fluid over thesame speed ranges as for FIG. 13.

FIG. 15 is a graph of electrical power consumption against flow for thesame fluid over the same speed ranges as for FIG. 13.

The common theme running through the first, second and third embodimentsdescribed thus far is the inclusion in the impeller of a taper or otherdeformed surface which, in use, moves relative to the adjacent housingwall thereby to cause a restriction with respect to the line of movementof the taper or deformity thereby to generate thrust upon the impellerwhich includes a component substantially normal to the line of movementof the surface and also normal to the adjacent internal pump wall withrespect to which the restriction is defined for fluid locatedtherebetween.

In order to provide both radial and axial direction control qt least oneset of surfaces must be angled with respect to the longitudinal axis ofthe impeller (preferably at approximately 45° thereto) thereby togenerate or resolve opposed radial forces and an axial force which canbe balanced by a corresponding axial force generated by at least oneother tapered or deformed surface located elsewhere on the impeller.

In the forms thus far described top surfaces of the blades 8, 207 areangled at approximately 45° with respect to the longitudinal axis of theimpeller 100, 204 and arranged for rotation with respect to the internalwalls of a similarly angled conical pump housing. The top surfaces ofthe blades are deformed so as to create the necessary restriction in thegap between the top surfaces of the blades and the internal walls of theconical pump housing thereby to generate a thrust which can be resolvedto both radial and axial components.

In the examples thus far the bottom faces of the blades 8, 207 comprisesurfaces substantially lying in a plane at right angles to the axis ofrotation of the impeller and, with their deformities define a gap withrespect to a lower inside face of the pump housing against which asubstantially only axial thrust is generated.

Other arrangements are possible which will also, relying on theseprinciples, provide the necessary balanced radial and axial forces. Sucharrangements can include a double cone arrangement where the conical topsurface of the blades is mirrored in a corresponding bottom conicalsurface. The only concern with this arrangement is the increased depthof pump which can be a problem for in vivo applications where sizeminimisation is an important criteria.

Fourth Embodiment

With reference to FIG. 18 a further embodiment of the invention isillustrated comprising a plan view of the impeller 300 forming part of a“channel” pump. In this embodiment the blades 301 have been widenedrelative to the blades 207 of the third embodiment to the point wherethey are almost sector-shaped and the flow gaps between adjacent blades301, as a result, take the form of a channel 302, all in communicationwith axial cavity 303.

A further modification of this arrangement is illustrated in FIG. 19wherein impeller 304 includes sector-shaped blades 305 having curvedleading and trailing portions 306, 307 respectively thereby definingchannels 308 having fluted exit portions 309

As with the first and second embodiments the radial and axialhydrodynamic forces are generated by appropriate profiling of the topand bottom faces of the blades 301, 305 (not shown in FIGS. 18 and 19).

FIG. 20 illustrates a perspective view of an impeller 304 which followsthe theme of the impeller arrangement of FIGS. 11 and 19 in perspectiveview and where like parts are numbered as for FIG. 19. In this case thefour blades 305 are joined at mid-portions thereof by a blade support inthe form of a conical rim 350 and have edge portions which are shaped soas to have an increased curvature on the trailing edge 351 thereofcompared with the leading edge 352.

Fifth Embodiment

A fifth embodiment of a pump assembly according to the inventioncomprises an impeller 410 as illustrated in FIG. 21 where, conceptually,the upper and lower surfaces of the blades of previous embodiments areinterconnected by a top shroud 411 and a bottom shroud 412. In thisembodiment the blades 413 can be reduced to a very small width as thehydrodynamic behaviour imparted by their surfaces in previousembodiments is now given effect by the profiling of the shrouds 411, 412which, in this instance, comprises a series of smooth-edged wedges withthe leading surface of one wedge directly interconnected to the trailingedge of the next leading wedge 414.

As for previous embodiments the top shroud 411 is of overall conicalshape thereby to impart both radial and axial thrust forces whilst thebottom shroud 412 is substantially planar thereby to impartsubstantially only axial thrust forces.

It is to be understood that, whilst the example of FIG. 21 shows thesurfaces of the shroud 411 angled at approximately 45° to the vertical,other inclinations are possible extending to an inclination of 0° to thevertical which is to say the impeller 410 can take the form of acylinder with surface rippling or other deformations which impart thenecessary hydrodynamic lift, in use.

With reference to FIGS. 22 to 24 a specific example of the conceptembodied in FIG. 21 is illustrated and wherein like components arenumbered as for FIG. 21.

It will be observed that, with reference to FIG. 24, the blades 413 arethin compared to previous embodiments and, in this instance, are arcuatechannels 416 therebetween which allow fluid communication from a centrevolume 417 to the periphery 418 of the impeller 410.

In this arrangement it will be noted that the wedges 419 are separatedone from the other on each shroud by channels 419. The channels extendradially down the shroud from the centre volume 417 to the periphery418.

In such designs with thin blades, the magnets required for the drivingtorque can be contained within the top or bottom volute or both, alongwith the optional soft magnetic yokes to increase rotor efficiency.

A variation of this embodiment is to have the wedge profiling cut intothe inter surfaces of the housing and have smooth shroud surfaces.

Sixth Embodiment

In contrast to the embodiments illustrated with respect to FIGS. 3A, 3Band 3C an arrangement is shown in FIG. 25 wherein the “deformed surface”comprises a stepped formation 510 forming part of an inner wall of thepump housing (not shown). In this instance the rotor including blade 511includes a flat working surface 512 (and not having a deformed surfacetherein) which is adapted for relative movement in the direction of thearrow shown with respect to the stepped formation 510 thereby togenerate hydrodynamic thrust therebetween.

Seventh Embodiment

With reference to FIG. 26 there is shown an arrangement of rotor blade610 with respect to stepped formation 611 and wherein the rotor blade610 includes a deformed surface 612 at a working face thereof. In thisinstance the deformation comprises curved edges 613, 614. As for theprevious embodiment relative movement of the rotor blade 610 in thedirection of the arrow with respect to deformed surface 611 forming partof the pump housing (not shown) causes relative hydrodynamic thrusttherebetween.

The foregoing describes principles and examples of the presentinvention, and modifications, obvious to those skilled in the art, canbe made thereto without departing from the scope and spirit of theinvention.

Principles of Operation

With particular reference to FIG. 27 this specification describes thesuspension of an impeller 600 within a pump housing 601 by the use ofhydrodynamic forces. In this specification the suspension of theimpeller 600 is performed dominantly which is to say exclusively byhydrodynamic forces.

The hydrodynamic forces are forces which are created by relativemovement between two surfaces which have a fluid in the gap between thetwo surfaces. In the case of the use of the pump assembly 602 as arotary blood pump the fluid is blood.

The hydrodynamic forces can arise during relative movement between twosurfaces even where those surfaces are substantially entirely parallelto each other or non-deformed. However, in this specification,hydrodynamic forces are caused to arise during relative movement betweentwo surfaces where at least one of the surfaces includes a “deformedsurface”.

In this specification “deformed surface” means a surface which includesan irregularity relative to a surface which it faces such that, when thesurface moves in a predetermined direction relative to the surface whichit faces the fluid located in the gap there between experiences a changein relative distance between the surfaces along the line of movementthereby to cause a hydrodynamic force to arise therebetween in the formor a thrust force including at least a component substantially normal tothe plane of the gap defined at any given point between the facingsurfaces.

In the example of FIG. 27 there is a first deformed surface 603 formingat least part of a first face 604 of impeller 600 and a second deformedsurface 605 on a second face 606 of the impeller 600.

The inset of FIG. 27 illustrates conceptually how the first deformedsurface 603 may form only part of the first face 604.

The first deformed surface 603 faces first inner surface 607 of the pumphousing 601 whilst second deformed surface 605 faces second innersurface 608 of the pump housing 601.

In use first gap 609 defined between first deformed surface 603 andfirst inner surface 607 has a fluid comprising blood located thereinwhilst second gap 610 defined between second deformed surface 605 andsecond inner surface 608 also has a fluid comprising blood locatedtherein.

In use impeller 600 is caused to rotate about impeller axis 611 suchthat relative movement across first gap 609 between first deformedsurface 603 and first inner face 607 occurs and also relative movementacross second gap 610 between second deformed surface 605 and secondinner surface 608 occurs. The orientation of the deformities of firstdeformed surface 603 and second deformed surface 605 relative to theline of movement of the deformed surfaces 603, 605 relative to the innersurfaces 607, 608 is such that the fluid in the gaps 609, 610experiences a change in height of the gap 609, 610 as a function of timeand with the rate of change dependant on the shape of the deformities ofthe deformed surfaces and also the rate of rotation of the impeller 600relative to the housing 601. That is, at any given point on either innersurface 607 or 608, the height of the gap between the inner surface 607or 608 and corresponding deformed surface 603 or 605 will vary with timedue to passage of the deformed surface 603 or 605 over the innersurface.

Hydrodynamic forces in the form of thrust forces normal to the line ofrelative movement of the respective deformed surfaces 603, 605 relativeto the inner surfaces 607, 608 thus arise.

With this configuration it will be noted that the first gap 609 lipssubstantially in a single plane whilst the second gap 610 is in the formof a cone and angled at an acute angle relative to the plane of thefirst gap 609.

Accordingly, the thrust forces which can be enlisted to first gap 609and second gap 610 are substantially normal to and distributed acrossboth the predominantly flat plane of first deformed surface 603 andnormal to the substantially conical surface of second deformed surface605 thereby permitting restoring forces to be applied between theimpeller 600 and the pump housing 601 thereby to resist forces whichseek to translate the impeller 600 in space relative to the pump housing601 and also to rotate the impeller 600 about any axis (other than aboutthe impeller axis 611) relative to the pump housing 601. Thisarrangement substantially resists five degrees of freedom of movement ofimpeller 600 with respect to the housing 601 and does so predominantlywithout any external intervention to control the position of theimpeller with respect to the housing given that disturbing forces fromother sources, most notably magnetic forces on the impeller due to itsuse as rotor of the motor are net zero when the impeller occupies asuitable equilibrium position. The balance of all forces on the rotor,effected by manipulation of magnetic and other external sources, may beadjusted such that the rotor is predominantly hydrodynamically born.

It will be observed that these forces increase as the gaps 609, 610narrow relative to a defined operating position and decrease as the gaps609, 610 increase relative to a defined operating gap. Because of theopposed orientation of first deformed surface 603 relative to seconddeformed surface 605 it is possible to design for an equilibriumposition of the impeller 600 within the pump housing 601 at a definedequilibrium gap distance for gaps 609, 610 at a specified rotorrotational speed about axis 611 and rotor mass leading to a closeapproximation to an unconditionally stable environment for the impeller600 within the pump housing 601 against a range of disturbing forces.

Characteristics and advantages which flow from the arrangement describedabove and with reference to the embodiments includes:

-   -   1. Low haemolysis, hence low running speed and controlled fluid        dynamics (especially shear stress) in the gap between the casing        and impeller. This in turn led to the selection of radial        off-flow and minimal incidence at on-flow to the rotor;    -   2. Radial or near-radial off-flow from the impeller can be        chosen in order to yield a “flat” pump characteristic (HQ)        curve.

INDUSTRIAL APPLICABILITY

The pump assembly 1, 200 is applicable to pump fluids such as blood on acontinuous basis. With its expected reliability it is particularlyapplicable as an in vivo heart assist pump.

The pump assembly can also be used with advantage for the pumping ofother fluids where damage to the fluid due to high shear stresses mustbe avoided or where leakage of the fluid must be prevented with a veryhigh degree of reliability—for example where the fluid is a dangerousfluid.

1.-17. (canceled)
 18. A rotary blood pump for use in a heart assistdevice, the pump having an impeller suspended in use within a pumphousing by hydrodynamic thrust forces generated by relative movement ofthe impeller with respect to and within the pump housing, and wherein atleast one of the impeller and the housing includes at least a firstdeformed surface lying on at least part of a first face which, in use,moves relative to respective facing surfaces on the other of theimpeller and the housing thereby to form a relatively moving surfacepair which generates relative hydrodynamic thrust between the impellerand the housing which includes a localized thrust componentsubstantially normal to the first deformed surface, and wherein theimpeller and the housing have a magnetic bearing to provide orsupplement the hydrodynamic thrust forces.